Tilted Fiber Grating Research Articles

A Simple All-Fiber Solc Filter Based on 45°-Tilted Fiber Gratings

A simple all-fiber Solc filter (AFSF), which consists of two 45°-tilted fiber gratings (45°-TFGs) UV-inscribed in the polarization-maintaining fiber (PMF) and a series of PMF cavities, is proposed and demonstrated. The performance of the proposed filter has been theoretically simulated and experimentally verified. Both the simulated and experimental results show that the bandwidth of the filter could be tuned by the PMF sub-cavity length and the number of PMF cavities. And the free spectral range (FSR) only depends on the sub-cavity length. As a proof of that, the bandwidths of AFSF with different number of PMF sub-cavity (N=2, N=3, N=4) and the same PMF sub-cavity length of 30cm are 4 nm, 2.6nm, and 2nm, respectively. The FSRs of 3-stage AFSF with different PMF sub-cavity length (L=20 cm and L=40 cm) are 15.3 nm and 7.97 nm, respectively. Furthermore, we have also investigated the tunability of the AFSF by controlling the temperature of PMF cavity with a tuning sensitivity around 1.205 nm/°C. Compared with existing fiber-optic Solc filters, the AFSF with prominent advantages such as extremely simple and robust structure, thermal tunability in wavelength, and low cost will bring a bright future for applications in optical communication and sensing systems.

Label-free and specific detection of soluble programmed death ligand-1 using a localized surface plasmon resonance biosensor based on excessively tilted fiber gratings.

Programmed death ligand-1 (PD-L1) plays an important role in tumor evasion from the host immune system. The level of soluble PD-L1 (sPD-L1) in serum is closely related to tumor aggressiveness and outcomes. This study aimed to propose a localized surface Plasmon resonance (LSPR) biosensor based on excessively tilted fiber grating (ExTFG) coated with large-sized (∼160 nm) gold nanoshells for label-free and specific detection of sPD-L1. The experimental results showed that the limit of detection (LOD) of the immunosensor for sPD-L1 in buffer solutions was ∼1 pg/mL due to the enhanced LSPR effect resulting from the interaction between sPD-L1 molecules and anti-sPD-L1 monoclonal antibodies. The detection of sPD-L1 in complex serum media, such as fetal bovine serum, confirmed that the label-free immunosensor was extremely specific to sPD-L1 and could identify it at a concentration as low as 5 pg/mL. Therefore, it can be potentially applied in clinic for the fast and early diagnosis of cancer.

In-fiber single-polarization diffraction grating based on radiant tilted fiber grating.

In this Letter, we propose an in-fiber single-polarization diffraction grating based on a radiant tilted fiber grating (TFG), in which the s-polarization light could be diffracted from the fiber core to free space with the wavelength-dependent diffraction angle. For the first time, we have presented the theoretical model of angular dispersion of a radiant TFG by employing the Fourier optics analysis method. The angular dispersion of the radiant TFG has been numerically characterized in terms of tilt angle, period, and wavelength. In the experiment, we have measured the diffraction angles and angular dispersion of the radiant TFGs with tilt angles of 41°, 45°, and 47° UV-inscribed into single-mode fiber, where the experimental results matched well with the simulation results. The simulation and experimental results have indicated that the diffraction light of radiant TFGs has a linear polarization state with over 0.99 degree of polarization at the tilt angle range from 41° to 47°.

Real-time access to the coexistence of soliton singlets and molecules in an all-fiber laser.

Recent progress in time-stretch spectroscopy accelerates the exploration of ultrafast dynamics in mode-locked lasers through mapping the spectral information into time domain. Here, we report on real-time access to the coexistence of soliton singlets and molecules in an all-fiber laser mode-locked by a 45° tilted fiber grating. By virtue of the dispersive Fourier transform process, spectral information of the pulse trains under multi-pulse states can be resolved. It is identified that soliton singlets and soliton molecules coexist in one cavity roundtrip with different assembling forms. In addition, consecutive recordings of the shot-to-shot spectra further enable insight into the transient dynamics of soliton molecules. Particularly, varieties of internal motions, including the diverging/oscillating phase evolution and the temporal separation vibration, are validated loosely bound intra-soliton molecules. All of these findings unveil the scenarios of multi-soliton phenomena in fiber lasers, as well as highlight the significance of pulse interaction towards both scientific research and practical applications.

Low-cost temperature- and strain-insensitive twist sensor based on a hybrid fiber grating structure.

We proposed and experimentally demonstrated a temperature- and strain-insensitive twist sensor based on a hybrid fiber grating structure, in which the hybrid grating structure is constructed with a 45°-tilted fiber grating and a chirped fiber Bragg grating UV-inscribed in a single-mode fiber in series. The sensing performance has been evaluated by experimental and numerical analyses, which are in good consistency. The experimental results show that the hybrid-grating-structure-based twist sensor has a maximum twist sensitivity up to 15.037dB/rad. Moreover, due to the invariability of the fiber birefringence and the state of polarization of the input light, such sensor has intrinsically low temperature and strain sensitivities of 7.86×10-3 dB/°C and 6.7×10-5 dB/με, corresponding to the maximum twist measurement error resulting from temperature and strain of 5.2×10-4 rad/°C and 4.5×10-6 rad/με, respectively.

Widely Wavelength-Tunable Mode-Locked Fiber Laser Based on a 45°-Tilted Fiber Grating and Polarization Maintaining Fiber

We present a passively mode-locked Erbium-doped fiber laser with tunable parameters including central wavelength, 3-dB bandwidth, and pulse duration. The mode-locking mechanism of the laser is realized by using single-walled carbon nanotubes (SWCNTs) polyvinyl alcohol composite film as a saturable absorber. The tunable operation is implemented via a fiber birefringence filter consisting of a polarization maintaining (PM) fiber and a Brewster fiber grating. The laser achieves a maximum spectral tuning range of 36 nm with 8-cm PM fiber. The maximum spectral width variation of 5.19 nm is acquired when the PM fiber is 12 cm. Simultaneously, the spectral widths of pulses at different central wavelengths are also adjustable. Furthermore, the total cavity length is 8.28 m, which is the shortest cavity length to obtain such wide tuning range in an Erbium-doped fiber laser based on SWCNTs.

Excessively tilted fiber grating based Fe3O4 saturable absorber for passively mode-locked fiber laser.

A novel approach to saturable absorber (SA) formation is presented by taking advantage of the mode coupling property of excessively tilted fiber grating (Ex-TFG). Stable mode-locked operation can be conveniently achieved based on the interaction between Ex-TFG coupled light and deposited ferroferric-oxide (Fe3O4) nanoparticles. The central wavelength, bandwidth and single pulse duration of the output are 1595 nm, 4.05 nm, and 912 fs, respectively. The fiber laser exhibits good long-term stability with signal-to-noise ratio (SNR) of 67 dB. For the first time, to the best of our knowledge, Ex-TFG based Fe3O4 SA for mode-locked fiber laser is demonstrated.

Pump-controlled flexible generation between dissipative soliton and noise-like pulses from a mode-locked Er-doped fiber laser.

We experimentally investigate the alternative generation of dissipative solitons (DSs) or noise-like pulses (NLPs) from a passively mode-locked Er-doped all-fiber laser with a 45° tilted fiber grating (45° TFG). The 45° TFG serves as an in-fiber polarizer for mode locking in the all-fiber laser cavity. Under the fixed orientations of the polarization controllers, flexible generation between DSs and NLPs can be precisely controlled by adjusting the pump power only. To the best of our knowledge, the total cavity length 4.92 m of our fiber laser is the shortest one among all NLP fiber lasers. We obtain the DS with 3 dB bandwidth of 20.4 nm centered at 1577 nm and the NLP with 3 dB bandwidth of 25.2 nm centered at 1574 nm. The fundamental repetition rate of the mode-locked pulses is 42.3 MHz. This DS-NLP switchable Er-doped fiber laser can be regarded as a multifunctional optical source for diverse practical and potential applications.

Excessively tilted fiber grating-based vector magnetometer.

A compact optic-fiber vector magnetometer is proposed and experimentally demonstrated, which is based on an excessively tilted fiber grating (Ex-TFG) assistant with the magnetic fluid (MF). Without any complicated processing, the cladding mode resonances of the bare Ex-TFG packaged by the MF show high sensitivity to slight perturbations by the magnetic field. Due to the excellent magneto-optical properties of the MF and the azimuth-dependent refractive index sensitivity of the Ex-TFG, such a magnetometer can achieve the magnetic field intensity sensitivity of 2.45nm/mT and the orientation sensitivity of 0.41nm/deg. In addition, based on the spectral interrogation, the detection limit of the magnetic field intensity could reach around 8.1μT at the minimum wavelength measurement accuracy of 0.02nm.

Hybrid Grating in Reduced-Diameter Fiber for Temperature-Calibrated High-Sensitivity Refractive Index Sensing

We propose and experimentally demonstrate a hybrid grating, in which an excessively tilted fiber grating (Ex-TFG) and a fiber Bragg grating (FBG) were co-inscribed in a reduced-diameter fiber (RDF). The hybrid grating showed strong resonances due to coupling among core mode and a set of polarization-dependent cladding modes. This coupling showed enhanced evanescent fields by the reduced cladding size, thus allowing stronger interaction with the surrounding medium. Moreover, the FBG’s Bragg resonance confined by the thick cladding was exempt from the change of the surrounding medium’s refractive index (RI), and then the FBG can work as a temperature compensator. As a result, the Ex-TFG in RDF promised a highly sensitive RI measurement, with a sensitivity up to ~1224 nm/RIU near the RI of 1.38. Through simultaneous measurement of temperature and RI, the temperature dependence of water’s RI is then determined. Therefore, the proposed hybrid grating with a spectrum of multi-peaks embedded with a sharp Bragg resonance is a promising alternative for the simultaneous measurement of multi-parameters for many RI-based sensing applications.

Graphene oxide-deposited tilted fiber grating for ultrafast humidity sensing and human breath monitoring

We propose and experimentally demonstrate a high-performance relative humidity (RH) sensor by depositing graphene oxide (GO) onto tilted fiber grating (TFG). The largely tilted grating planes of the employed TFG can induce a set of polarization-dependent cladding modes and strong evanescent field to couple with the humidity-dependent dielectric of GO layer. The GO-deposited TFG presents the response sensitivities of 18.5 pm/%RH and 0.027 dB/%RH in the range of 30%∼80%RH by tracking the wavelength and intensity of a specific cladding mode resonance. By monitoring the human breath with different frequencies, the sensor exhibits an ultrafast response within ∼42 ms due to the thin GO film and unimpeded permeation of water molecules through GO interlayer. The easy fabrication, low hysteresis, fast response, and high repeatability and reliability of the proposed RH sensor may enable many potential applications including pharmaceutical processing, human health and environmental monitoring.

Black phosphorus based fiber optic biosensor for ultrasensitive cancer diagnosis

We propose the first black phosphorus (BP) – fiber optic biosensor for ultrasensitive diagnosis of human neuron-specific enolase (NSE) cancer biomarkers. A novel optical-nano configuration has been exploited by integrating BP nanosheets with a largely tilted fiber grating (BP-TFG), where the BP is bio-functionalized by the poly-L-lysine acting as a critical cross-linker to facilitate bio-nano-photonic interface with extremely enhanced light-matter interaction. BP nanosheets are synthesized by a liquid ultrasonication-based exfoliation and deposited on fiber device by an in-situ layer-by-layer method. The BP-induced optical modulation effects in terms of thickness-tunable feature, polarization-dependence and enhanced light-matter interaction are experimentally investigated. The anti-NSE immobilized BP-TFG biosensor has been implemented to detect NSE biomarkers demonstrating ultrahigh sensitivity with limit of detection down to 1.0 pg/mL, which is 4 orders magnitude lower than NSE cut-off value of small cell lung cancer. The enhanced sensitivity of BP-TFG is 100-fold higher than graphene oxide or AuNPs based biosensors. We believe that BP-fiber optic configuration opens a new bio-nano-photonic platform for the applications in healthcare, biomedical, food safety and environmental monitoring.

All-fiber polarization interference filter based uniform multiwavelength erbium-doped fiber laser

We demonstrate a compact and stable room-temperature multiwavelength erbium doped fiber laser by employing 45° tilted fiber gratings based on an all-fiber polarization interference filter. Benefiting from the filter, the channel number, linewidth, uniformity and stabilization of the multiwavelength laser are greatly improved. The filter also works as a functional polarizing device in a nonlinear polarization rotation, leading to the multiwavelength operation. Instead of a highly nonlinear fiber, the single mode fiber is used to provide enough linearity and to reduce the splice loss. More than 80 wavelengths (within a 3 dB bandwidth), lasing with a linewidth of 0.03 nm and a signal-to-noise ratio of 33 dB, are obtained. The wavelength spacing of 0.164 nm agrees with the channel spacing of the filter, and can be flexibly controlled by adjusting the length of the filter.

Kelly sideband suppression and wavelength tuning of a conventional soliton in a Tm-doped hybrid mode-locked fiber laser with an all-fiber Lyot filter

We demonstrate a stable conventional soliton in a Tm-doped hybrid mode-locked fiber laser by employing a homemade all-fiber Lyot filter (AFLF) and a single-wall carbon nanotube. The AFLF, designed by sandwiching a piece of polarization-maintained fiber (PMF) with two 45° tilted fiber gratings inscribed by a UV laser in PMF with a phase-mask scanning technique, shows large filter depth of ∼9 dB and small insertion loss of ∼0.8 dB. By optimizing the free spectral range of the AFLF, the Kelly sidebands of a conventional soliton centered at 1966.4 nm can be dramatically suppressed without impairing the main shape of the soliton spectrum. It gives the pulse duration of 1.18 ps and bandwidth of 3.8 nm. By adjusting the temperature of the PMF of the AFLF from 7°C to 60°C, wavelength tunable soliton pulses ranging from 1971.62 nm to 1952.63 nm are also obtained. The generated soliton pulses can be precisely tuned between 1971.62 nm and 1952.63 nm by controlling the temperature of the AFLF.

Study on polarization spectrum and annealing properties of 45°-tilted fiber gratings

We have investigated the whole polarization-extinction-ratio (PER) spectrum and annealing properties of 45°-tilted fiber gratings (45°-TFGs). Experimental results show the PER spectrum of 45°-TFGs is a Gaussian-like profile and covers a 540 nm bandwidth from 1260 to 1800 nm, in which the bandwidth with PER greater than 10 dB is over 250 nm. The output polarization distribution of 45°-TFGs was analyzed by employing a bulk linear polarizer, and the results show a perfect figure “8”, which indicates that the 45°-TFG is a type of linear polarizer. Moreover, the annealing property of 45°-TFGs was measured up to 700°C, in which the PER of the grating started to decrease at 300°C and reached the minimum at 700°C. Based on these results, the 45°-TFGs can be used as an ultra-wide bandwidth in-fiber polarizing device.

Numerical and Experimental Characterization of Radiation Mode of 45° Tilted Fiber Grating

In this paper, we have numerically and experimentally investigated the radiation mode of 45° tilted fiber grating (45° TFG). By introducing the axial distributions into the volume current method, we have established a three-dimensional theoretical model to analyze spatial distributions of the radiation mode of 45° TFG, and measured the radiation mode spatial distributions in experiment. Both numerical and experimental results show that the radiation mode of 45° TFG exhibits an exponential reduction along the fiber axial direction, and a quasi-Gaussian profile along the radial direction of fiber. Additionally, we have also measured the detailed polarization-dependent azimuthal intensity distributions of radiation modes of both s- and p-polarized core modes. The degree of polarization of radiation mode is around 99.886%, which is independent of the length and the polarization extinction ratio of the grating. Moreover, for the first time, we have experimentally observed a very weak p-polarized component existing in the radiation mode, which has matched well with the simulation results.

基于倾斜光纤光栅的连续可调谐锁模激光器

基于倾斜光纤光栅的连续可调谐锁模激光器

Theoretical analysis of tuning property of the graphene integrated excessively tilted fiber grating for sensitivity enhancement

The graphene integrated optical fiber devices show many excellent characteristics, especially their tunable optical properties. Here we investigate thoroughly the influence of the tuning property of the graphene on the resonance and sensing performance of the excessively tilted fiber grating (Ex-TFG) operating around the dispersion tuning point. An improved piecewise discretization method for the finite-difference mode solver combined with coupling mode theory is presented to explore the general variation rule of mode characteristics and polarization-dependent resonances corresponding to polarized TE/TM0,m and HE/EHv,m modes of the graphene integrated Ex-TFG. The results reveal that both p-polarized and s-polarized modes and their resonances are greatly influenced by the tunable graphene. On this basis, the sensitivity enhancement of the graphene coated Ex-TFG is explored in detail. It is shown that the sensing performance of both polarized cases can be greatly improved by tuning the chemical potential. In particular, the graphene induces a greater influence on both resonance wavelength and resonance strength of the s-polarized mode, but the p-polarized one has a higher sensitivity. We believe that these unique tuning properties make the graphene integrated Ex-TFG devices ideal for the telecommunication and sensing applications, such as tunable fiber modulators and biochemical sensors.

Immunosensing platform with large detection range using an excessively tilted fiber grating coated with graphene oxide.

We report an immunosensing platform with a large detection range using an excessively tilted fiber grating (ExTFG) coated with graphene oxide (GO). ExTFG was inscribed in standard single-mode fiber; GO film was coated on the fiber surface through hydrogen bond. The effectiveness and uniformity of GO deposited on the ExTFG surface were investigated by field emission scanning electron microscopy and energy spectrum method. Bovine serum albumin (BSA) monoclonal antibodies (MAbs) were used as biometric units to link the GO film through a covalent bond for the specific detection of BSA, so as to evaluate the performances of the proposed biosensor. The whole dynamic immobilization process of BSA MAbs and BSA detection were observed by the spectral evolution of the sensor. Experimental results show that the fabricated GO-coated ExTFG biosensor has a large detection range from 1.5nM-75nM and fast response for BSA antigen; the limit of detection is ∼0.88 nM by using an optical spectrum analyzer with a resolution of 0.03nm, and the dissociation constant KD and the affinity constant KA are calculated to be ∼6.66×10-9 M and ∼1.5×108 M-1, respectively. The proposed GO-coated ExTFG immunosensing platform could lay a foundation for the specific detection of other biomolecules.

Wavelength-tunable Q-switched fiber laser based on a 45° tilted fiber grating

A continuously tunable Q-switched all-fiber Er-doped laser based on a 45° tilted fiber grating and tunable bandpass filter is demonstrated. The 45° tilted fiber grating is used to achieve the nonlinear polarization rotation (NPR) along with two polarization controllers (PCs), Q-switching is realized due to the fact that the NPR effect induced intensity-dependent loss. Under the pump power of 655 mW, the Q-switched optical spectrum can be continuously tuned from 1512 nm to 1552 nm by simply rotating the tunable bandpass filter. During the wavelength tuning process, the average output power increases from 0.282 mW to 4.884 mW while the repetition rate enhances from 27.3 kHz to 119 kHz. To the best of our knowledge, this is the widest continuously tunable range of Q-switched fiber Lasers based on nonlinear polarization rotation effect and spectral bandpass filter.